1. Field of the Invention
This invention relates to an apparatus for reading out a radiation image stored on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to stimulating rays which cause it to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light. This invention particularly relates to a radiation image read-out apparatus wherein preliminary read-out is conducted for approximately ascertaining the image input information prior to final read-out for reproducing a visible image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in the applicant's Japanese Unexamined Patent Publication No. 56(1981)-11395 and U.S. Pat. No. 4,258,264, it has been proposed to use a stimulable phosphor in a radiation image recording an reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored thereon, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a very wide range in proportion to the amount of energy stored thereon, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it to an electric image signal to reproduce a visible image on a recording medium or a display device.
This system is also advantageous in that after the radiation image information stored on the stimulable phosphor sheet has been read out and converted into an electric image signal, the electric image signal can then be easily processed in the manner most appropriate for obtaining a radiation image suitable for viewing, particularly for diagnostic purposes, when it is used for reproducing a visible image on a photographic light-sensitive material or on a display device such as a CRT.
As mentioned above, in the radiation image system using a stimulable phosphor sheet, compensation for deviation of the level of the radiation energy stored on the stimulable phosphor sheet from a desired level can easily be carried out by adjusting the read-out gain to an appropriate value when photoelectrically reading out the light emitted by the stimulable phosphor sheet upon stimulation thereof. Therefore, the quality of the reproduced radiation image is not adversely affected by a fluctuation in radiation dose due to fluctuating tube voltage or MAS value of the radiation source, a variation in the sensitivity of the stimulable phosphor sheet or the photodetector, a change in radiation dose according to the condition of the object, or a fluctuation in the radiation transmittance according to the object, and the like. Also, it is possible to obtain a desirable radiation image even when the radiation dose to the object is low.
However, in order to eliminate various effects caused by the fluctuation of radiographic exposure conditions and to obtain a radiation image having high image quality, it is necessary to ascertain the image input conditions of the radiation image stored on the stimulable phosphor sheet and the image input pattern which is determined by the portion of the body (e.g. the chest or the abdomen) or the radiographic method used, such as plain image or contrasted image radiographic, before reproducing the radiation image as a visible image for viewing, and then to adjust the read-out gain appropriately based on the detected input conditions and the image input pattern. The image input conditions and the image input pattern will hereinafter be simply referred to as the image input information when they are referred to generically. It is also necessary to determine the scale factor to optimize the resolution according to the contrast of the image input pattern.
Investigation of the image input information can be conducted prior to the visible image reproduction by use of the method as disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the disclosed method, a read-out operation for detecting the image input information of a radiation image stored on a stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is conducted in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes (hereinafter referred to as the final read-out), and thereafter the final read-out gain is adjusted, and/or an appropriate signal processing is conducted, and/or the scale factor is adjusted to an appropriate value on the basis of the image input information obtained by the preliminary read-out.
For carrying out this method there can be used a radiation image read-out apparatus including a final read-out system comprising a means for emitting stimulating rays to a stimulable phosphor sheet carrying a radiation image stored thereon, a means for moving the stimulable phosphor sheet and a light detection means for photoelectrically detecting light carrying the radiation image and emitted by the stimulable phosphor sheet upon exposure to stimulating rays; a preliminary read-out means comprising a means for emitting stimulating rays, a means for moving said stimulable phosphor sheet and a light detection means for photoelectrically detecting the light carrying the radiation image and emitted by said stimulable phosphor sheet upon exposure to stimulating rays, the stimulation energy of which is lower than that of the stimulating rays in the final read out; and control means for setting the read-out conditions and/or the signal processing conditions on the basis of the image input information obtained by the preliminary read-out. Apparatuses of this type are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 58(1983)-67242 and 58(1983)-67243.
As described above, the level of the stimulating rays used in the preliminary read-out is lower than the level of the stimulating rays used in the final read-out. That is, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out should be lower than the effective energy of the stimulating rays used in the final read-out. In order to make the level of the stimulating rays used in the preliminary read-out lower than the level of the stimulating rays used in the final read-out, the output of the stimulating ray source such as a laser beam source may be decreased in the preliminary read-out, or the stimulating rays emitted by the stimulating ray source may be attenuated by an ND filter, an AOM or the like positioned on the optical path. Alternatively, a stimulating ray source for preliminary read-out may be provided independently of the stimulating ray source for the final read-out, and the output of the former may be made lower than the output of the latter. Or, the beam diameter of the stimulating rays may be increased, the scanning speed of the stimulating rays may be increased, or the moving speed of the stimulable phosphor sheet may be increased in the preliminary read-out.
By the aforesaid method, since the image input conditions and the image input pattern of a radiation image stored on the stimulable phosphor sheet can be ascertained in advance, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy by adjusting the read-out gain and the scale factor on the basis of the detected image input information and by processing the detected electric image signal in the manner most suitable for the image input pattern without using a read-out system having a wide dynamic range.
However, when the aforesaid preliminary read-out is conducted, the time required for reading out the radiation image and processing the detected electric image signal is naturally longer than in the case where only the final read-out is conducted. Also, when the final read-out conditions are determined on the basis of the results of the preliminary read-out as in the aforesaid method, subtle differences in contrast tend to arise among the reproduced radiation images of different objects even when the radiation images relate to the same body portion of the objects. This is disadvantageous because when radiation images of the same body portion of different objects are diagnosed in comparison with each other it is generally preferable from the doctor's point of view for all of the radiation images to have the same contrast. It is also preferable to maintain the contrast of the reproduced radiation images at a prescribed level in the case where the performance of a radiation image recording apparatus is to be tested by recording radiation images of special phantoms on stimulable phosphor sheets and then examining the visible radiation images reproduced from these sheets. On the other hand, differently from when radiation images of portions of the human body are reproduced for diagnostic purposes, in the case of reproducing radiation images of microtomic organ sections or portions of animal bodies for research purposes, there is generally no problem even if the radiation images are reproduced with fixed contrast.